Halo: The Master Chief Collection is set to hit the shelves next week, and as hype is starting to build up, we get to catch a glimpse of the story line of Halo 5: Guardians from a leaked cut-scene.
The cut-scene in question is part of the Master Chief collection preview, which is unlocked during gameplay.
The relevance of this scene to the main plot of Halo 5: Guardians, is unclear, so I guess we have to wait till next year to find out.